The present invention relates to a metal halide lamp (hereinafter, referred to as an MHL) and more particularly, to an MHL capable of improving luminous efficiency and color rendering by improving the structure of a luminous tube.
Generally, lighting lamps of high brightness and long life are installed street light fixtures and industrial work areas. Among such commercially available lamps, there are a high pressurized mercury lamp, a high pressurized sodium lamp and a MHL. The mercury lamps are the most widely used, and their lifetime is comparatively long. However, their luminous efficiency is not so good and their luminous color feels chilly and cold. The sodium lamps are best in view of the luminous efficiency but their color rendering characteristic is not so good. However, the MHL is better than the mercury lamp in view of the luminous efficiency, and is best in view of the color rendering. Accordingly, the use of the MHL style is becoming more and more widespread. The cost of the MHL, however, is slightly high in cost but should be solved in the near future. Together with the increase of use of the MHL, some prerequisites to be met follow naturally. Particularly, in the field of the interior decoration in which the illumination effects play an important role, such prerequisites should be satisfied with carefulness. Particularly, a small MHL which is used in the field of interior decoration should have low power consumption, high efficiency, high color rendering and a long lifetime. Here, the MHL which is chiefly used in an interior room will be described below.
In FIG. 1, one example of a horizontal start type MHL is shown among the above conventional MHLs. Referring to FIG. 1, in an oval crystal luminous tube 1 are opposedly provided a pair of electrodes 2a and 2b. In each electrode is formed a zirconia temperature-keeping layer 3. Also, in the space of luminous tube 1 are sealed and filled with predetermined rare gases, mercury and metal halide. An outer tube encloses luminous tube 1 and its accessories. The inner portion of outer tube 4 is sealed and filled with nitrogen and inert gases. Socket connectors 5 are provided in both sides of outer tube 4, and are electrically connected with electrodes 2a and 2b. Here, a reference numeral 6 represents a getter which absorbs the remaining gas and increases the vacuum.
FIG. 2 shows a conventional partly extracted luminous tube of the lamp shown in FIG. 1. Referring to FIG. 2, luminous tube 1 has an oval shape, at both ends on the long axis of which are provided electrodes 2a and 2b. From each electrode is drawn out a lead wire W. Particularly, as shown in FIG. 2, between the lead wire and the lead wire is installed a molybdenum thin plate M for maintaining a gas-tight seal, thereby connecting the lead wires. Also, as described above, on either electrode is formed temperature-keeping layer 3, which prevents the lowering of the temperature in both electrodes. On the other hand, an arch portion represented by dotted curved lines which is located between the electrodes shows an upward bending phenomenon of an arc generated due to the inner temperature difference in the luminous tube during the illumination to be described later.
FIG. 3 is a cross-sectional view of the luminous tube shown in FIG. 2, cut along line III--III. As shown in FIG. 3, the cross-section of luminous tube 1 has a cylindrical shape having a predetermined thickness. Due to the temperature difference by generation of the minimum-temperature portion during the illumination of the luminous tube to be described later, the arc is bent upwards by the bending phenomenon.
By the way, in such a conventional MHL of the above structure, if a state of the lamp is investigated during the illumination, the lower end of luminous tube 1 is cooled by a convection phenomenon of the gas in luminous tube 1 and becomes a relative minimum-temperature portion. Also, by the temperature difference due to such a convection, the arc is bent upwards. Accordingly, crystal luminous tube 1 is degraded by non-uniform local heating. On the other hand, the vapor pressure of the metal halide is varied depending upon the temperature of the minimum-temperature portion. Accordingly, condensation of the compound is generated by the cooling action in the lower end of the luminous tube, as a result that the sufficient vapor pressure is not formed. As a result, the efficiency of the lamp is lowered.